1. Field of the Invention
The present invention is related to the assessment of testability and maintainability of a complex system and, more particularly, to an automated system for receiving estimates of failure rates and maintenance task times for individual modules and automatically calculating testability and maintainability parameters of a system composed of the modules.
2. Description of the Related Art
Organizations involved in the development of complex systems typically require that failures in the system be identifiable and repairable within a given amount of time. In determining whether these system level parameters are met, assumptions of the failure rate, testing time and replacement time are developed for each of the modules or replaceable items in the system and a series of calculations are required to be performed to generate the values of the testability and maintainability parameters. One example of the type of parameters and calculations required to determine the value of those parameters is set forth in PROCEDURE V of the standard MIL-HDBK-472, incorporated herein by reference, which was issued by the Department of Defense in January, 1984.
As noted in the MIL-HDBK-472 standard, PROCEDURE V can be applied to virtually any type of equipment including mechanical equipment, although the standard was issued for application to avionics, ground and shipboard electronics. In controlling the development of such complex systems, it is desirable to verify, to the extent possible, that the equipment can be maintained in a timely fashion. Rather than permitting the designers of the system to "pull numbers out of the air," PROCEDURE V of MIL-HDBK-472 standard requires that actual or estimated data be provided for each of the primary replaceable items, including an identification of all known failure modes of each of the replaceable items, the failure rate associated with that failure mode, the maintenance procedure used to remove and replace each replaceable item and the fault detection/isolation test strategy for the system.
The failure modes identify categories of failures which are known as possibly occurring in the replaceable items. Taking a simple example, a temperature-controlled switch, such as a thermostat controlling a heater, can fail high so that the heater runs continuously or can fail low so that the heater remains off regardless of the temperature. These are two possible failure modes of a temperature-controlled switch. The failure rates associated with these modes represent how often each type of failure can be expected to occur. The values of the failure rates would be based upon actual experience if the same temperature-controlled switch has been used in other applications or similar switches have been used in similar applications. In this manner, an evaluation of the modes of failure and the rates of failure associated with each mode is provided for each module or replaceable item in, e.g., a complex avionics system as an initial step of meeting the requirements of PROCEDURE V.
The basic fault detection and isolation outputs test strategy described in PROCEDURE V of MIL-HDBK-472 only describes a single set of tests directly connected to termination nodes identifying a replaceable item and having a failure rate and a maintenance activity block associated therewith. The activity block identifies the maintenance procedure. Each step in the maintenance procedure is referred to as an elemental task and the average or mean time required to perform the elemental task is estimated to provide information for time line analysis.
PROCEDURE V of the MIL-HDBK-472 standard provides a large number of formulas to calculate maintainability parameters such as mean time to repair (MTTR); average repair time for each replaceable item; percent isolation to a single replaceable item and to a group of replaceable items; mean maintenance man hours per repair, per maintenance action, per operating hour and per flight hour (in the case of avionics); and the, e.g., ninety-fifth percentile of the corrective maintenance times. However, PROCEDURE V only contemplates manual calculation of these parameters and no attempt is made to minimize redundant steps or provide efficient calculation techniques. During the development of a system, the parameters ideally should be recalculated as additional information is gained regarding the characteristics, i.e., failure modes, failure rates, elemental task times, etc., of the modules to verify that the system requirements are being met. Repeated iteration of the calculations is not addressed except by providing for both approximate (PROCEDURE V.A) and detailed (PROCEDURE V.B) calculations. By requiring excessive time and effort, PROCEDURE V virtually guarantees that calculations will be performed as few times as possible.
In addition, PROCEDURE V.B. does not address the subject of iterative replacement. Under iterative replacement, if three modules were suspect, the first module would be replaced and tests run to see if the system failure was fixed. If not, the second suspect module is replaced and repair is verified. The third module would be replaced only if the failure was still not fixed. The alternative to iterative replacement is group replacement. For group replacement the maintainer always replaces all three suspect modules, then verifies that the failure has been fixed. Since it is most likely that only one of those three modules actually has a fault, much wasted effort is incurred in handling and attempting to fix the two good modules. Notwithstanding the increased logistics cost, elementary probability shows that iterative replacement is superior to group replacement in any system with reasonable self-testing ability, such as modern complex electronic systems.
Furthermore, the equations presented in PROCEDURE V.B are wholly inadequate in addressing iterative replacements. In addition, there are many other useful parameters which can be obtained from the data required for PROCEDURE V and more parameters can be obtained with the addition of data that can be relatively easily supplied in addition to the required data.